1. Field of the Invention
The present invention relates to a method of forming a metal layer using atomic layer deposition and a semiconductor device having the metal layer as a barrier metal layer or the upper or lower electrode of a capacitor.
2. Description of the Related Art
As the integration density of semiconductor devices increases, high dielectric materials having a large dielectric constant have been developed to obtain large capacitance in a small area. For example, a BST (BaSrTiO.sub.3) film having a perovskite crystal structure has a dielectric constant of about several hundreds through one thousand in a bulk state, which is different to a silicon nitride film, a silicon oxy-nitride film and a tantalum oxide (Ta.sub.2 O.sub.5) film which are conventionally used for a capacitor. A BST film is advantageous in that a thin dielectric film can be implemented such that an equivalent oxide thickness is less than 10 .ANG. even when the thickness of the BST film is more than 500 .ANG.. An electrode such as platinum (Pt) which is not oxidized may be used for a BST electrode. An electrode such as ruthenium (Ru) or iridium (Ir), which holds the characteristics of a conductor even if it is oxidized and forms oxide ruthenium (RuO.sub.2) or oxide iridium (IrO.sub.2), may also be used for a BST electrode.
To obtain a high dielectric BST film having excellent capacitance and leakage current characteristics, a thermal process needs to be performed at a high temperature after depositing BST film. At this time, a barrier metal layer needs to be formed to prevent oxidation of an ohmic layer and a polysilicon plug due to diffusion of oxygen. The barrier metal layer is interposed between the polysilicon plug and a lower electrode.
Conventionally, a titanium nitride (TiN) film is usually used for the barrier metal layer, but the TiN film is oxidized at a temperature of more than 450.degree. C. When a high temperature thermal process is performed in an oxygen atmosphere after depositing a BST film, a TiN film and a polysilicon plug are oxidized because platinum (Pt) lets oxygen easily pass through. Especially, a non-conductive TiO.sub.2 film is formed when the TiN film is oxidized. In addition, platinum (Pt) and silicon (Si) is diffused into the TiN film, and thus the TiN film cannot act as a barrier metal layer. It is known that the diffusion of Pt and Si is caused by the columnar structure of TiN. Accordingly, it is necessary to restrain the diffusion of oxygen by implementing an amorphous structure which dose not have a grain boundary acting as a path of diffusion.
From this necessity, compounds containing a refractory metal have been studied. A barrier metal layer formed of a compound containing a refractory metal has a problem that adjustability and reproducibility of composition is decreased when the compound is deposited by chemical vapor deposition, due to the complexity of the composition. Accordingly, a reactive sputtering process is usually performed in a nitrogen atmosphere when forming a barrier metal layer of a compound containing a refractory metal. However, a barrier metal layer formed by a sputtering process has a poor step coverage so that it cannot be suitable for a barrier metal layer in a capacitor, the structure of which becomes more complex as the integration density of a semiconductor device increases, for example, a barrier metal layer which is formed at the lower portion of a trench having a high aspect ratio in a trench type capacitor.